With recent advances in fermentation and metabolic engineering, fermentation routes to various products have been identified and developed (Clomburg, Appl Microbiol Biotechnol, 86: 419-434, 2010; Peralta-Yahya, Biotechnol J, 5: 147-162, 2010; Cho, Biotechnol Adv, pii: S0734-9750(14)00181-5, 2014. However, all of these fermentation routes are energy (ATP)-consuming or, at best, energy (ATP)-neutral, which restricts product yield in energy-limited systems and uncouples product production from microorganism growth. The present invention provides energy (ATP)-generating pathways that overcome these limitations by providing novel fermentation routes and pathways to a variety of products, including acids, alkenes, aldehydes, alcohols, and diols. These pathways are directly coupled to microorganism growth and offer high product yields.
In particular, the invention relates to fermentation pathways involving Ptb-Buk. Phosphate butyryltransferase (Ptb) (EC 2.3.1.19) natively catalyzes the reaction of butanoyl-CoA and phosphate to form CoA and butanoyl phosphate. Butyrate kinase (Buk) (EC 2.7.2.7) natively catalyzes the reaction of butanoyl phosphate and ADP to form butyrate (butanoate) and ATP. Accordingly, these enzymes together (Ptb-Buk) natively catalyze the conversion of butanoyl-CoA to butyrate and generate one ATP via substrate level phosphorylation (SLP).
The inventors have discovered that Ptb is promiscuous and is capable of accepting a variety of acyl-CoAs and enoyl-CoAs as substrates, such that Ptb-Buk may be used to convert a number of acyl-CoAs and enoyl-CoAs to their corresponding acids or alkenates, respectively, while simultaneously generating ATP via substrate level phosphorylation.
Furthermore, in combination with an aldehyde ferredoxin oxidoreductase (AOR) and an alcohol dehydrogenase, acids formed via the Ptb-Buk system can be further converted to their respective aldehydes, alcohols, or diols. AOR (EC 1.2.7.5) catalyzes the reaction of an acid and reduced ferredoxin (which can, for example, be generated from oxidation of CO or hydrogen) to form an aldehyde and oxidized ferredoxin. Alcohol dehydrogenase (EC 1.1.1.1 and EC 1.1.1.2) can convert an aldehyde and NAD(P)H to an alcohol and NAD(P).
Introduction of Ptb-Buk and/or AOR into a heterologous species, therefore, provides a novel, alternate route to the formation of native and non-native products, such as acids, alkenes, ketones, aldehydes, alcohols, and diols at high yields, thus overcoming limitations of the current state of the art.